High saturated fat diet, low fruit and vegetable (F/V) intake, and sedentary lifestyle characterize a majority of adult Americans, heightening risk for cardiovascular disease and cancers. Improved diet and activity can reduce risk, but the best prescription to promote healthier diet and more active lifestyle simultaneously remains unknown. By comparing 4 groups randomized to perform all combinations of increasing healthy versus decreasing unhealthy eating and activity, this research tests competing hypotheses about how to optimize simultaneous health behavior change. The Familiarity Hypothesis predicts that the most familiar dieting prescription (decrease fat, increase physical activity) will maximize healthy behavior change. The Optimal Substitution Hypothesis, based on Behavioral Economic Theory, predicts that increasing F/V intake while decreasing sedentary behavior will surpass alternatives by maximizing behavioral substitution of healthful for unhealthful eating and activity. The Low Inhibitory Demand Hypothesis, based on Self-Control Theory, predicts that increasing F/V's while increasing physical activity will be most successful because this prescription places fewest demands on self-control resources. Community-dwelling adults (n=200) with inactive lifestyle and suboptimal diet will self-monitor diet, physical activity, and mood via PDAs during: a 2- week baseline, a 3-week prescription period (when payment is contingent upon changing eating and activity simultaneously to targeted standards), and a 4-month maintenance period. Targeted and collateral diet and activity changes will be measured by self-report, accelerometer, and grocery receipts. Bogus pipeline urinary testing will encourage adherence. Laboratory testing will measure behavioral choices, craving, and attentional allocation to restricted foods and activities in a permissive context in order to shed light on behavioral and psychological processes that mediate healthy lifestyle change. Findings will help to fill an important gap in clinical knowledge about how to optimize healthy simultaneous change in diet and activity among adults.